# Question: tribonacci numbers are the numbers in a sequence in which...

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Tribonacci numbers are the numbers in a sequence in which the first three elements are 0, 1, and 1, and the value of each subsequent element is the sum of the previous three elements: 0, 1, 1, 2, 4, 7, 13, 24, ...

Write a user defined function to determine the nth Tribonacci number. Name the function Tribonacci(). The function takes an integer n ≥ 1 and returns its corresponding Tribonacci number. For example, the function must return 0 for n=1, 1 for n=2 and 13 for n=7 and so on.

The function must display an error message if the number entered was either less than 1 or not an integer.

The function must have a help section that shows what it exactly does, its input and its output. Use the function to determine the Tribonacci number for n = 0, 2.5, 3, 10, 20, and 34.